Packerhead concrete pipe making machines have rotatable packerheads that are moved in an upward direction in molds to form concrete pipes. Conveyors operate to move the concrete from hoppers into the molds above the packerheads. The amount of concrete in the molds above the packerheads is related to the torque required to rotate the packerheads. The operating speeds of the conveyors are used to control the supply of concrete in the molds. An apparatus for forming a concrete pipe having a control to adjust the rate of concrete supplied by the conveyor to the mold is disclosed by G. E. Hand in U.S. Pat. No. 4,406,605. In this apparatus the horsepower of the drive motor for the packerhead is sensed and used to adjust the operating speed of the conveyor to control the rate of concrete moved into the mold. There is time delay in adjusting the operating speed of the conveyor which can cause soft spots in the pipe formed in the mold. This control does not modulate the lift rate of the packerhead to self compensate for overpack and underpack conditions.
F. Gauger in U.S. Pat. No. 3,746,494 discloses a method and apparatus for making concrete pipe having uniform compaction. The torque supplied by the packerhead motor during the formation of the pipe varies with the rate of supply of concrete. This torque signal is used to decrease the rate of rise of the packerhead when there is a decrease in the torque and increase the rate of rise of the packerhead when there is an increase in the torque signal. Unset concrete is supplied to the mold at essentially a constant rate.
T. A. Adly in U.S. Pat. No. 4,639,342 describes a combined concrete feed and packerhead lift control for a packerhead concrete pipe making machine to produce a pipe having substantially uniform density throughout the length of the pipe. The rate of concrete directed into the mold is controlled by varying the operating speed of the conveyor motor in response to the packerhead motor load signal. This is the primary control for maintaining uniform density of the concrete in the pipe. Upper and lower limits of the packerhead motor load signal are selected to identify permissible limits of overpacking and underpacking of the concrete. The lift speed of the packerhead is increased when the motor load signal exceeds the upper limit and decreased when the motor load signal falls below the lower limit. The lift speed remains substantially constant when the packerhead motor load signal is between the upper and lower limits.